1. Field of the Invention
The present invention relates to a method for preventing surfaces of a Zn-based metallic coating from rusting, and a rust preventive. Particularly, it relates to a rust preventive agent and a method for preventing surfaces of a base, such as metallic articles and metallic materials including steel plates used as parts or materials in manufacture of automobiles, building and so forth, coated with a Zn-based metal from rusting.
2. Description of the Prior Art
Plated steel plates, which are used as a material in automobiles, building, etc., such as steel plates plated with zinc or zinc-based alloy are used after painting. The plated steel plates are passed through a variety of steps before being painted; therefore they are left unpainted in the course of these steps for a considerably long time. So, while they are left unpainted, rust is generated or various substances are adsorbed or adheres, on the surfaces of the plated steel plates. As a result, for example, the adhesion of a paint applied in the subsequent step may be poor.
Thus, heretofore, the surfaces of plated steel plates have been subjected to chromate treatment as a primary rust prevention treatment. The corrosion resistance obtained by the chromate treatment is about 48 hours in the salt spray test defined in JIS Z-2371. However, the coating film obtained by the chromate treatment contains hexavalent chromium. It is known that the hexavalent chromium causes an allergy (chromate ulcer) when adhering a skin, and also it is pointed out that the hexavalent chromium is suspected to be a carcinogen (carcinoma cutaneum). So, a primary rust prevention treatment not using hexavalent chromium is required.
Meanwhile, as a primary rust prevention treatment other than the chromate treatment there is proposed a method using a coating agent for metal surfaces, which agent is comprised of carboxyl-modified epoxy resin or polyvinyl butyral resin containing a silica. The use of the coating agent results in a corrosion resistance equivalent to or higher than that obtained by the chromate treatment, but the appearance of the resulting coated metal surface is colorless or metallic. However, it is recently required that the metal surface coated by a primary rust prevention treatment has a black or blackish appearance so as to give a feeling of quality.